Electrolytic fabrication of sulfoxides

ABSTRACT

ORGANIC SULFOXIDES ARE MANUFACTURED BY ANODICALLY OXIDIZING THE CORRESPONDING ORGANIC SULFIDES IN AN AQUEOUS ORGANIC SOLVENT AND ELECTROLYTE AND BY MAINTAINING THE ANODE POTENTIAL BETWEEN 0.4 AND 1.7 VOLTS.

Apfil30,1974 QTHIBAULT ETAL 3,808,112

ELECTROLYTIC FABRICATION or SULFOXIDES Original Filed nay 1; 1969 I N VEN TORS CL 4005 779 /5401 7 United States Patent Oifice 1 f 7 3,808,112 H ELECTROLYTIC FABRICATION F SULFOXIDES Claude Thibault, Pau, and Pierre Mathieu, Arthez-de- 'Bearn, France, assignors to Societe Nationale des Petroles dAquitaine, Courbevoie, France Continuation of abandoned application Ser. No. 820,803, May 1, 1969. This application Dec. 6, 1971, Ser. No. 205 351 v r Int. Cl. C07b 3/00; C07c 147/00; B011: 1/00 US. Cl. 204-79 13 Claims ABSTRACT OF THE DISCLOSURE Organic sulfoxides are manufactured by anodically oxidizing the corresponding organic sulfides in an aqueous organic solvent and electrolyte and'by maintainmg the anode potential between 054 and 1.7 volts.

This is 'a continuation of application Ser. N0. 820,803, filed May 1, 1969, now abandoned.

" 1 The new process according to the present invention has been developed with a view to the electrolytic production of various organic sulphoxides, especially dialkyl or diaryl sulphoxides; it is particularly useful for the manufacture of dimethyl sulphoxide, an industrial product for which numerous practical applications have been found during the past decade. v I

3 Th e anodic oxidation of organic sulphides is'a known operation and is capable of producing sulphoxides, as shown by Fichter and Braun in Berichte 47, 1526-34, and Pursley, in US. Pat. No. 3,200,054. Nevertheless, the operating procedures so far used led to results which industrially were of little interest. The sulphoxide, when obtained in an acceptable yield, necessitated a heavy expenditure for electricity, because the coulomb eflici'ency (Fara- 'day yield) of the preparation was insufiicient'. On the .otherhand, the sulfoxide was generally obtained together i with large'proportio'ns of other oxidized compounds, particularly the corresponding sulphone. Thus, for example,

the first authors referred to above obtained diphenyl sulphi'oxide with a molar yield of 82.5% with respect to the diphenyl sulphide and with'a coulomb efiiciency'of only about 62% Pursley describes a process which gives mainly the sulphorie, accompanied by a proportion of sulphoxide f not exceeding The present invention provides a truly industrial process by which it is possible to produce an organic sulphoxide, by anodic oxidation of the corresponding sulphide,.with

' yields, both 'as regards weight and electrical, which are close or equal to 100%. In addition, the new process can xi be carried into efiect easily and economically, as it does not require any electrolyte or costly solvent, and can 'be "-eifected in conventional electrolytecells, even withoutdiaphragms; this last possibility provides the advantages of reducing the energy consumption. The new process offers perfect working safety, because of the absence of peroxidic products,'in contrast with the chemical processes for the production of sulphoxides. Due 'to'the fact that the oxidation selectivity results'in practically only sulphoxide being produced, separation of the resultant product is considerably facilitated as compared with the known processes.

' The invention results firstly from the discovery that it "is not 'suflicient to regulate the temperature and the con- 3,808,112 Patented Apr. 30, 1974 formed aloneand with a very high electrical yield. Other features of the invention which enable excellent results to be obtained are set out hereinafter in' the present description.

In the process according to the invention, a solution 7 of organic sulphide in an organic solvent containing water and a dissolved electrolyte is subjected to electrolysis between insoluble electrodes, the solvent and the anion of the electrolyte being selected from those of which the oxi dation potential is decidedly higher than that of the sulphide, and the anode is maintained at a potential higher than that of the sulphide but lower than that of the solvent {and of the anion.

Since generally the sulphoxides have oxidation potentials higher than the corresponding sulphides, it is expedient for the potential of the anode to be regulated, when carrying the new process into elfect, to a value lower than that which corresponds to the oxidation of the formed sulphoxide; in this way, the production of more oxidized compounds, particularly sulphone, is avoided. 1

Although the oxidation potentials can vary with the nature of the treated sulphides, they are generally of the order of +0.4 to 1.7 volts on carbon or platinum anodes. In general, the preferred range of anodic potential according to the invention is approximately +0.5 to +1.6 volts. It is obvious that the potential to be applied has to be re-adjusted according to the nature of the anode being used. For example, in the case where dimethyl sulphide is oxidized into dimethyl sulphoxide with carbon or platinum anodes, the anode potential is usually from about +0.8 to

, +1.1 in dimethyl formamide and can reach +1.6 in acetonitrile. The potentials which are indicated in the present specification are understood to be by comparison with the saturated calomel electrode.

The anodic oxidation according to the invention can be represented by the summary reaction:

in which the hydrocarbon groups R and R are like or different and can be such as alkyl, aryl, cycloalkyl al'karyl, aralkyl, optionally substituted with other groups,

such as, for example, halogen, hydroxyl and carboxyl groups. Thus, for example, each of the groups R and R can'be a methyl, ethyl, propyl, isopropyl, n-butyl, is0- butyl, tert-butyl, hexyl, decyl, dodecyl, eicosyl, etc. group, or phenyl, tolyl, cresyl, ethyl-phenyl, dodecylphenyl,

I chlorophenyl, cyclohexyl, cyclopentyl, naphthyl or other group.

As previously indicated, the solvent, which plays an important part, is selected from the liquids of which the other hand, with a certain proportion of water added thereto, it must show a dielectric constant sufficient to lead to the necessary ionic dissociation of the electrolyte.

' Bearing these conditions in mind, it is easy to find var- ,j ious appropriate solvents, among which can be mentioned,

for example, the dialkyl formamides and particularly dimethyl formamide, acetonitrile, nitromethane, etc.

centration of the electrolytic bath, as well. as the current V been done according'to the prior art, for obtaining the best possible result. The applicants have found that an important factor which has to be taken into account is the anodic potential, which has to be kept within certain density and the voltage at the terminals of the cell, as has I K nitrates, sulphonates, or others, of the alkali and/or well-established limits in order that the sulphoxide is According to one preferred feature of the invention,

the solvent is itself a sulphoxide and best of all, actually the sulphoxide which is to be manufactured. By way of example, dimethyl sulphoxide constitutes an excellent solvent which is applicable for the manufacture of various dialkyl or diaryl sulphoxides, and more especially,

the manufacture of dimethyl sulphoxide.

Suitable as electrolyte are the different mineral salts, such as halides, halogenates, perhalogenates, sulphates,

alkaline earth metals, of which the anion is oxidized at a potential decidedly higher than the oxidation potential of the treated organic sulphide; the acids in which the than also satisfies this condition are likewise suitable. It is understood that the choice of the suitable electrolyte depends on the nature of the sulphide which isto be oxidized.

' For carrying the new process into elfect, the anode of the electrolytic cell can be platinum, gold, nickel, palladium, cobalt, rhenium, iridium, tantalum, carbon, or other materials which cannot be attacked under the working conditions. Alloys of the various aforementioned metals can be used. Carbon, preferably in the form of graphite, can with advantage be impregnated in accordance with the conventional electro-chemical technique. The anode can also be formed of lead dioxide.

" As the nature of the cathode is of less importance than that or the anode, metals such as iron, stainless steel, lead, mercury, copper, etc. are suitable, in addition to the materials indicated above for the anode, with the exception of lead dioxide. Obviously, since the cathode must remain insoluble, the choice thereof depends on the nature of the electrolyte being used.

When the electrolytic cell is arranged for working without a diaphragm, it is necessary to choose a cathode on which the sulphoxide is not reduced.

The concentration of organic sulphide in the liquid medium to be electrolyzed can vary considerably; with sulphides suchas dimethyl sulphide, it is advantageous to work in relatively concentrated solutions, for example, with a content by weight of 5 to 50% or better still, from to 40%. i

It is generally preferable for the molar ratio between the water which is present and the sulphide initially introduced to be equal to or higher than 0.45, and particularly from 0.45 to 20.

The process according to the invention is preferably carried out at moderate temperatures, especially between 0 and 100 C. For relatively volatile sulphides, it is expedient to operate, for example, between 10 and 35 C.; excellent results are obtained with dimethyl sulphide at ambient temperature, that is to say, between approximately 20 and 30 C. In the case of heavier sulphides, as, for example diphenyl sulphide, it is advantageous to raise the temperature for increasing the conductivity of the medium.

After the anodic oxidation of the sulphide, the sulphoxide is separated from the liquid medium by any known means, for example, by distillation and/or crystallization, .etc.

- In the particular case of dimethyl tremely advantageous-form of the new process consists in carrying out the oxidation of the dimethyl sulphide in solution in dimethyl sulphoxide.

:. Although the metals referred to above are quite suitable in the case of dimethyl sulphoxide, platinum and carbon are particularly advantageous when the electrolyte .is acid; acids such as sulphuric, perchloric, nitric or toluosulphonic acids are very suitable.

. As regardsthe cathode, in the particular case referred process for the manufacture of dimethyl sulphoxide is a cell without a diaphragm, having one or more graphite anodes and one or more stainless steel or platinum cathodes. The cell has an agitation system and a cooling arrangement which permits the temperature of the electrolytie med um te be k pt b ow 373 Q, wh ch sulphoxide (:CH SO, which industrially is very important,one exp 4 represents the boiling point of dimethyl sulphide. A device is preferably associated with the cell for recovering the sulphide entrained by the hydrogen liberated at the cathode. This device comprises one or more known means, such as condensers or adsorption or absorption columns. According to a preferred feature of this invention, the recovery device is formed by a dimethyl sulphoxide washer; the sulphide dissolved by this device can be directly reintroduced, with its solvent, into the electrolysis cell.

The electrolysis can be continued until complete exhaustion of the sulphide initially introduced with the electrolytic liquid. However, in order to avoid a considerable drop in intensity, it may be advantageous to interrupt the electrolysis when only a part of the sulphide is oxidized, to separate the products and to recycle the remaining sulphide in the electrolytic cell. It is also possible periodically to extract a portion of the electrolyzed liquid, in order to separate therefrom the sulphoxide which has formed and to replace it with fresh sulphide, before all the initial sulphide is oxidized. I

The separation of the products of the electrolytic mixture can be effected by any known means, with or without neutralization of the electrolyte.

All or certain of these operations can be efiected continuously.

The non-limiting examples which follow illustrate the invention, by reference to the accompanying drawings.

FIG. 1 is a vertical axial section of the electrolysis cell which is used, along the line II of FIG. 2.

FIG. 2 is a top plan view of the same cell.

EXAMPLE 1 17 g. of dimethyl sulphide g. of water 46 g. of sodium paratoluosulphonate 900 ml. of dimethyl formamide.

The operation is carried out in a cylindrical cell 1 (see FIGS. 1 and 2)'with a capacity of about ml. Three anodes A1-AZ-A3, formed by small graphite plates or grids of platinum wire, are disposed in the form of a U at the centre of the cell and electrically connected together. The cathodes C1 and C2 are grids of stainless steel or platinum wire; the first cathode C1, in the form of a U, is placed inside the U formed by the anodes A1-A2-A3, while the second is an arc of a circle concentric with the walls of the cell 1. The two cathodes are connected to one another. The circular cathode C2 encloses the anode branches A1 and A3, but not the transverse branch A2, behind which is disposed the reference electrode 4 for measuring the anode potential. The agitation is assured by a magnetic agitator 5. The water bath optionally provided for cooling the cell is not shown. The total surface of the anodes A1, A2 and A3 is 64 cm?, and that of the cathodes C1 and C2 together isv 75 cm.

The anode potential is measured in known mannerin relation to the saturated calomel electrode 4. The contact with the solution of the cell is assured by a bridge containing a saturated solution of KNO in the same solvent as that of the electrolized medium, especially dimethyl formamide or dimethyl sulphide. The end of the bridge is formed by a Luggin capillary 6 coming into contact with the anode A2.

The anode potential of the lateral branches A1 and A3, which is generally slightly stronger than on A2, because of the asymmetry of the cell, is determined in the same manner.

A direct current is caused to flow, the anode potential being +0.89 volt on A2 and +1.04 volts for A1 and A3. The current density is 0.43 A./dm. The temperature of the solution is from 20 to 25 C. When 42.5% of the theoretical quantity of electricity has passed through the solu w the e ectrolysis is stopped and the coulomb yield of the formed dimethyl sulphoxide is determined, after having measured the sulfoxide by chromatography: this electric yield is 100%. r v I EXAMPLE 2 I j Using the same cell as in Example 1, a solution of the same composition, but in which the electrolytezis-sulphuric acid having a concentration of 0.28 N, is elec+ trolyzed.

The anode potential is +0.91. v. forA2 and +1.09 vv.

on A1 and A3; the anodecurrent density is 0.48.A.(dm. the temperature is kept between 20 and 25C. After the passage of 80% of the theoretical current, the coulomb yield of the formation of dimethyl sulfoxide is found to be 100%.

EXAMPLE 3 In a cell without a diaphragm, equipped with an anode consisting of a platinum grid and a stainless steel cathode, a solution having the following composition is electrolyzed:

Solvent Dimethyl sulphoxide.

Dimethyl sulphide 215 g. per 1. of solution. Water 100 g. per l. of solution. H 80 43.5 g. per 1. of solution.

EXAMPLE 4 A solution having the same composition as in Example 3 is electrolyzed in a similar cell, in which the platinum anode is replaced by a carbon anode. The anode potential is +0.88 v. for A2 and +1.03 v. for A1 and A3; the temperature is kept at 23 -27 C. The voltage at the terminals of the cell varies between 7 and 10 volts and the current between 1.5 and 2.1 A.; the current density reaches 3.3 A./dm. After passage of 41.5% of the theoretical current, a coulomb yield of 95% of dimethyl sulphoxide is obtained.

EXAMPLE 5 Using the same cell and the same conditions as regards anode potential and temperature as in Example 4, a solution with the following composition is electrolyzed:

Solvent Dimethyl sulphoxide. Dimethyl sulphide 340 g. per litre of solution. H O 45 g. per litre of solution. H SO, 43.5 g. per litre of solution.

The anode potential on A2 is +0.88 v. and on A1 and A3 is +1.035 v. The voltage at the terminals varies from 8 to 8.5 v. and the current from 0.8 to 0.9 A., and hence the mean current density is 1.33 A./dm. After passage of 45.4% of the theoretical current, the coulomb yield is found to be 96.2% of DMSO.

EXAMPLE 6 Using a cell without a diaphragm and equipped with carbon anodes and platinum cathodes, a solution of the following composition is electrolyzed:

Solvent Dimethyl formamide. Diethyl sulphide 126 g. per litre of solution. H O 50 g. per litre of solution. H SO 43.5 g. per litre of solution.

The temperature is kept between and 28 C. and the anode potential is kept at a value from +0.835 to +0.98 vl, relative to the saturated calomel electrode. Under these conditions, the potential difference at the terminals of the cell varies from 3 to 5 v. and the cursulphoxide is obtained.

" I EXAMPLE 7 --Using the same cell as in Example 6, a solution having thefollowing composition is electrolyzed:

S01vent, Dimethyl formamide.

Diphenyl sulphide 140 g. per litre of solution. H O 50 g. per litre of solution. H 43.5 g. per litre of solution.

The anode potential is from +0.835 to +0.965 v. relative to the saturated calomel electrode. The temperature is kept between 22 and 27 C. The potential difference at the terminals varies between 2.2 and 3.5 v. and the current strength between 0.1 and 0.2 A., the mean anode current density being 0.25 A/dmP.

After passage of 61% of the theoretical current, the coulomb yield of diphenyl sulphoxide is raised to 94%.

The examples illustrate the anodic oxidation under atmospheric pressure, but it is possible to work under a higher pressure, in which case, the temperature can be higher than that at which the treated sulphide boils.

We claim:

1. A process for the manufacture of an organic sulfoxide consisting essentially of anodically oxidizing the corresponding organic sulfide of the formula R S-R wherein R and R are individually selected from the group consisting of unsubstituted and halogen-, hydroxyand carboxy-substituted alkyl, aryl, cycloalkyl, alkaryl and aralkyl groups in an aqueous organic solvent selected from the group consisting of dialkyl formamide, aliphatic nitrile, nitroalkane and organic sulfoxide, and an electrolyte selected from the group consisting of an acid, and a halide, halogenate, perhalogenate, sulfate, nitrate or sulfonate of an alkali metal or alkaline earthmetal between insoluble electrodes at a temperature between 0 and C., wherein said anode is carbon or platinum and the anode potential between 0.4 and 1.7 volts.

2. Process according to claim 1 wherein the sulfide is dimethyl sulfide.

3. A process for the manufacture of an organic sulfoxide which comprises anodically oxidizing the corresponding organic s'ulfide in an aqueous organic solvent selected from the group consisting of dialkylformamide, aliphatic nitrile, nitroalkane and organic sulfoxide, and an electrolyte selected from the group consisting of an acid, and a halide, halogenate, perhalogenate, sulfate, nitrate or sulfonate of an alkali metal or alkaline earth metal between insoluble electrodes at a temperature between 0 and 100 C., wherein the anode is carbon or platinum and the anode potential is between 0.4 and 1.7 volts.

4. Process according to claim 3 wherein the electrolysis is interrupted when 40% to 80% of sulfide has been oxidized, the'organic sulfoxide product is separated from the electrolyzed mixture and the remaining unoxidized sulfide is subjected to additional anodic oxidation.

5. Process according to claim 3 wherein the anode potential is between 0.5 and 1.6 volts.

6. Process according to claim 3 wherein the anode potential is about 0.8 to 1.1 volts.

7. Process according to claim 3 wherein the temperature is between 10 and 35 C.

8. Process according to claim 3 wherein the sulfide is dimethyl sulfide, the temperature is between 10 and 35 C., and wherein the anode potential is about 0.8 to 1.1.

9. Process according to claim 3 wherein the organic sulfide to be oxidized is of the formula R -SR wherein R and R are individually selected from the group consisting of unsubstituted and halogen-, hydroxyand 7 carboxy-substituted alkyl, aryl, cycloalkyl, alkaryl and aralkyl groups.

10. Process according to claim 9 wherein-the sulfide is dimethyl sulfide.

11. Process according to claim 3 wherein the mixture 5 8 References Cited UNITED STATES PATENTS 3.2011054 8/1965 'Pursley 204 79 3,418,224 12/1968 Bennett et a1. 204--78 OTHER REFERENCES Chemische Berichte, Fichter et al., 45, pp. 1376-1383 (1912). 1

10 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

